You choose how many targets Bogardan Hellkite’s triggered ability has and how the damage is divided as you put the ability onto the stack. Each target must receive at least 1 damage.

11/17/2017

You can’t deal damage with Bogardan Hellkite’s triggered ability to both a player and a planeswalker that player controls. You also can’t deal damage to more than one planeswalker controlled by the same player. If you choose to redirect the damage being dealt to a player to a planeswalker, you must redirect all the damage to a single planeswalker.

11/17/2017

If some of the creatures or players are illegal targets as Bogardan Hellkite’s triggered ability tries to resolve, the original division of damage still applies and the damage that would have been dealt to the illegal targets is lost. It won’t be dealt instead to a legal target.